1. Field of the Invention
The present invention relates generally to a device for transferring toner image from a photoconductive drum to a recording sheet in an imaging apparatus, and particularly to a device for determining and adjusting a transfer voltage in the imaging apparatus.
2. Description of the Related Art
In electro-photographic printing process, toner is transferred from a photoconductive drum to another media by bringing the media in contact with a toner image on the photoconductive drum. Then a voltage (also known as transfer voltage) is applied between the photoconductive drum and the media that causes toner that is charged to move from surface of the photoconductive drum to surface of the media. This transfer of the toner from the photoconductive drum to the media takes place either by a single transfer or a dual transfer. In a single transfer system, the media is most frequently paper. In a dual transfer system, the first transfer is from the photoconductive drum to an intermediate media, for example, an intermediate transfer belt, and the second transfer is from the intermediate media to print media.
The transfer voltage that is required for efficient transfer of the toner depends on number of factors that vary in the printing process. These factors include environmental temperature and humidity, paper resistivity, transfer roller resistivity, photoconductive drum thickness, etc. If the transfer voltage is either too high or too low, this leads to poor transfer of the toner from the photoconductive drum to the print media. In order to determine a proper setting for the transfer voltage, a sequence of measurements of voltages is done at the beginning of a print job. This sequence of measurements of the voltages is called a transfer servo process.
The transfer servo process comprises a number of sequential steps. Through these sequential steps, a range of voltages are applied to a transfer nip being formed between the photoconductive drum and a transfer roller that is positioned opposite to the photoconductive drum. Thereafter, a current resulting from the application of the voltages is tested to see if the resulting current is greater than or less than a target current level. The target current level is typically eight micro amps. A primary object of applying a range of voltages is to find a servo voltage that produces a current that most nearly matches the target current level. Once the servo voltage is found, it is further used to determine the transfer voltage.
Since a broad range of voltages needs to be investigated, therefore, a coarse search is first done to determine an approximate servo voltage followed by a fine search to refine the determination of the servo voltage. The coarse search starts with a low voltage and voltage is then increased in large voltage steps until the target current level is exceeded. Then, the fine search is performed that starts at a voltage that is below the last coarse voltage and the voltage is increased in small voltage steps until the target current level is exceeded. The coarse search typically includes up to 40 voltage steps and the fine search typically includes up to ten voltage steps, where each step takes about 25 milliseconds. The time to do the resulting search, i.e., coarse search and the fine search is variable and can take up to 1.25 seconds. For a printer running at 50 pages per minute, the coarse search and the fine search would take place over about 4 photoconductive drum revolutions. Therefore, it is desirable to reduce the number of photoconductive drum revolutions that take place outside of actual printing of a page. Reducing photoconductive drum revolutions outside of actual printing, results in longer life of the cartridges and the machines, along with a better print quality.
Additionally, the calculation of transfer voltage includes only one input, the servo voltage. The servo voltage responds proportionally to the resistance of the transfer nip. While the servo voltage serves as a leading indicator for optimum transfer voltage for the efficient transfer of the toner, there are a number of other indicators previously mentioned that affect the optimum transfer voltage. It is desirable to include such factors in the calculation of the transfer voltage. This would result in a more efficient estimate of the optimum transfer voltage given the availability of more information about the system.
Thus, there is a need to provide an apparatus and an algorithm for performing the transfer servo process, i.e., a process for the determination of the transfer voltage that will take less time and provide a more accurate determination of the transfer voltage that is needed for the efficient toner transfer at either the first or the second transfer. Additionally, there is a need to reduce the number of drum revolutions during this process.